1. Field of the Invention
The present invention relates to wireless telecommunications. More particularly, the present invention relates to a method and apparatus for signal acquisition in a wireless telecommunication system having large distance uncertainty between a terminal and hub.
2. Description of the Related Art
Digital wireless telecommunication systems interface multiple terminals to a hub via the use of digitally modulated radio frequency (RF) signals. This allows each terminal to provide telecommunication service to an individual user or subscriber without the need of wirebased connections. One example of a digital wireless telecommunication system that incorporates the hub and terminal architecture is a cellular telephone system. The hub in a cellular telephone system is referred to as the base station, and the terminals are referred to as mobile units or cellular telephones. In general, the digitally modulated RF electromagnetic signals used in a digital wireless telecommunication system are transmitted between the hub and terminals in a manner that allows various telecommunication services to be provided including audio based telephone service as well as fax and digital data transmission service. An RF signal transmitted from the hub to a terminal is referred to as the forward link signal, and an RF signal transmitted from a terminal to the hub is referred to as a reverse link signal.
Code division multiple access (CDMA) digital signal modulation is one example of a digital signal modulation technique that is employed within digital wireless telecommunication systems. In order for a wireless communication link to be established between a hub and a terminal utilizing CDMA modulated signals, the forward and reverse link signals must be "acquired" by the terminal and hub respectively. Signal acquisition is the process of synchronizing the state of a set of pseudorandom noise (PN) codes used to modulate the digital data before transmission with the state of another set of PN codes used to demodulate the data upon reception. The use of PN codes for modulating and demodulating data being transmitted is in accordance with CDMA technology. To acquire a forward link signal, a terminal performs multiple searches for a pilot channel carried by the forward link signal until the pilot channel is detected. To acquire a reverse link signal, the hub searches for access probes from any terminals during search windows that take place during the beginning of access slots that occur periodically at the hub. The access probes signify the beginning of a series of reverse link transmissions from a terminal. In general, a terminal becomes synchronized with the forward link signal before the hub becomes synchronized with the reverse link signal because the pilot signal from the hub is made available constantly, while access probes are only generated when communication with the associated terminal is being initialized.
In any wireless telecommunication system, transmissions between the hub and a terminal are delayed a certain amount because of the distance between the two systems. The amount of this delay will vary as the distance between the hub and terminal is modified by the movement of either system. This variation in delay can complicate the signal acquisition process since the exact amount of variation in delay is often unknown at a particular time. For the acquisition of the forward link signal, an unknown delay presents less of a problems because the terminal must already search in time to acquire the forward link signal, and, therefore, any time delay will be accounted for once the forward link signal is acquired. For the acquisition of the reverse link signal, however, the unknown delay is problematic because the access probe must arrive during the appropriate search window and have been modulated with a set of PN codes in the correct state in order to be detected. Therefore, any error in estimating the transmission delay will cause the access probe to go undetected. In cellular wireless telephone systems, the use of a search window at the hub is normally sufficient to compensate for the possible range of delays, as multiple searches for an access probe are performed during the search window, each offset in time. Since the range of distances between a hub and terminal in a land based cellular telephone system varies at most a by few miles, the delay uncertainty will be minimal and the use of a relatively small, or short, search window at the hub will ensure that the access probe is detected.
In satellite based wireless telecommunications systems employing the use of the hub and terminal architecture the RF signals associated with the forward and reverse links are reflected from a space based satellite during transmission. This makes the total distance traveled by the signals hundreds or thousands of miles. This increase in absolute distance also increases the variance in the distance between the hub and a terminal when compared to cellular telephone systems, which in turn increases the delay variation experienced by messages such as access probes transmitted from a terminal to the hub. In general, the delay variation is sufficiently large that the use of a search window at the hub of similar duration to that used in a digital wireless cellular telephone system is not sufficient to ensure the proper detection of an access probe. Therefore, a new method of performing signal acquisition is necessary for satellite based digital wireless telecommunications systems employing CDMA technology.
Despite the need for a new method of performing signal acquisition, it is preferable to implement and utilize as much existing cellular telephone equipment as possible in any new satellite based telecommunication system. Using existing cellular telephone equipment provides increased economy, and also allows one to incorporate new advances achieved in digital cellular telephone systems including higher efficiency and higher quality signal transmission. Thus, a method and apparatus for signal acquisition that allows existing cellular wireless telecommunications equipment to be used in the context of large delay uncertainty satellite based telecommunications system would be highly desirable. Additionally, because modifications in software can be implemented far more easily and with less expense than modifications in hardware, any new method for signal acquisition that allowed existing cellular wireless telecommunication systems or equipment to be used via modification of software would be more desirable still.